[delete-user-data] add note to FIRESTORE_DELETE_MODE about applicability (#53)

Author: rachelsaunders

delete-user-data/extension.yaml

@@ -106,8 +106,9 @@ params:
     type: select
     label: Cloud Firestore delete mode
     description: >-
-      How do you want to delete Cloud Firestore documents? To also delete documents in
-      subcollections, set this parameter to `recursive`.
+      (Only applicable if you use the `Cloud Firestore paths` parameter.) How do you want
+      to delete Cloud Firestore documents? To also delete documents in subcollections,
+      set this parameter to `recursive`.
     options:
       - label: Recursive
         value: recursive